1. Field of the Invention
The present invention generally relates to constant current circuits having MOS (Metal Oxide Semiconductor) structures, and more specifically, to technique proper for stabilizing operation of a circuit.
2. Description of the Related Art
In an analog circuit using a MOS (Metal Oxide Semiconductor) transistor, in order to stabilize operation, a reference voltage and a constant current source are critical. However, in the MOS transistor used for making the constant current source, unevenness of a threshold value of voltage in a manufacturing process or change of the threshold value of voltage due to temperature occurs. For example, if the threshold value of voltage becomes high, the constant current becomes great. If the threshold value of voltage becomes low, the constant current becomes small.
As a related art constant current circuit which corresponds to unevenness of the threshold value in the manufacturing process of the MOS transistor, a circuit shown in FIG. 1 is discussed in Japanese Patent No. 3517343.
FIG. 1 is a circuit diagram showing a structural example of a related art constant current circuit using an MOS (Metal Oxide Semiconductor) transistor. In a circuit shown in FIG. 1(a), a depression type MOS transistor (D-type MOS transistor) 31 and a resistance 32 are used. In a circuit shown in FIG. 1(b), only the depression type MOS transistor (D-type MOS transistor) 31 is used.
In the circuit shown in FIG. 1(b), a gate, a source, and a substrate of the D-type MOS transistor 31 are connected to a ground electric potential VSS, and a drain is connected to a high electric potential VDD. A constant current is caused to flow between the source and drain of the D-type MOS transistor 31.
In the circuit shown in FIG. 1(b) and using only the D-type MOS transistor 31, an absolute value of the constant current or a temperature coefficient is drastically changed due to change of the threshold value of voltage of the D-type MOS transistor 31 generated in a manufacturing process of the D-type MOS transistor 31 such as thermal diffusion, gate oxidization, or ion implantation.
On the other hand, in the circuit shown in FIG. 1(a), the resistance 32 is provided between the gate and source of the D-type MOS transistor 31. IN other words, the source of the D-type MOS transistor 31, the substrate, and one end of the resistance 32 are connected. The gate of the D-type MOS transistor 31 and another end of the resistance 32 are connected to the VSS. The drain of the D-type MOS transistor 31 is connected to the VDD.
Under this structure, due to unevenness of the manufacturing process, for example, if the threshold value of voltage of the D-type MOS transistor 31 becomes high, the constant current flowing in the D-type MOS transistor 31 is increased.
However, due to the voltage drop generated by the electric current flowing through the resistance 32 provided between the gate and source of the D-type MOS transistor 31, the gate electric potential of the D-type MOS transistor 31 is changed in a direction negative (minus “−”) against the source electric potential where the current is not constant. As a result of this, the constant current is stabilized.
On the other hand, if the threshold value of voltage of the D-type MOS transistor 31 becomes low, the constant current flowing in the D-type MOS transistor 31 is decreased. Since the voltage drop generated by the electric current flowing in the resistance 32 becomes small, the gate electric potential of the D-type MOS transistor 31 is changed in a direction positive (plus “+”) with the source electric potential where the constant current easily flows. As a result of this, the constant current is stabilized.
Generally, in a case where the temperature rises or a case where the constant current becomes large together with the change of the threshold value of voltage of the D-type MOS transistor 31, it is possible to obtain a more stabilized constant current by using the resistance 32 where the resistance value becomes larger, such as a poly-silicon resistor or diffusion resistor.
In addition, a structural example of a voltage reference circuit using the constant current circuit shown in FIG. 1(a) is discussed in Japanese Patent No. 3517343. A structural example of a voltage reference circuit using the constant current circuit shown in FIG. 1(b) is discussed in Japanese Laid-Open Patent Application Publication No. 9-325826 and Japanese Examined Patent Application Publication No. 4-65546.
FIG. 2 is a circuit diagram showing a structural example of a related art voltage reference circuit using the MOS (Metal Oxide Semiconductor) transistor. More specifically, FIG. 2 shows a structure example of a voltage reference circuit using the constant current circuit shown in FIG. 1(b) and discussed in Japanese Laid-Open Patent Application Publication No. 9-325826 and Japanese Examined Patent Application Publication No. 4-65546.
In the voltage reference circuit shown in FIG. 2, a drain of a depression type (D-type) n-channel MOS transistor 45 is connected to an electric power source at a high electric potential side. A source and a bulk of an enhancement type (E-type) n-channel MOS transistor 47 is connected to an electric power source at a low electric potential side.
A bulk and a source of the D-type n-channel MOS transistor 45 are connected to a drain of the E-type n-channel MOS transistor 47 at a connection point 48. Gates are connected to each other at a connection point 46 and the connection point 48. This connection point 48 is a voltage reference output where the electric power at the low electric potential is a standard electric potential.
Generally, an enhancement type (E-type) MOS transistor is a surface channel type transistor and therefore unevenness of a threshold voltage cause by manufacturing processes is small.
On the other hand, a depression type (D-type) transistor is an embedded channel type transistor and therefore unevenness of the threshold voltage caused by manufacturing processes is large. Hence, unevenness of a saturation drain electric current caused by manufacturing processes is extremely great.
FIG. 3 is a circuit diagram showing a structural example of a constant current circuit using the voltage reference circuit shown in FIG. 2.
In the constant current circuit shown in FIG. 3, a source of a depression type MOS transistor, namely a D-type MOS transistor 51 (indicated as “DepTr1” in FIG. 3) whose drain is connected to an electric power source 56 at the high electric potential side, a drain of an enhancement type MOS transistor, namely a E-type MOS transistor 53 (indicated as “EnhTr1” in FIG. 3) whose source is connected to a low electric potential side (ground), and gates are connected so that the voltage reference circuit shown in FIG. 2 is formed and a standard voltage 57 is obtained.
In addition, the standard voltage 57 is connected to the gate of an E-type MOS transistor 55 (indicated as “EnhTr 3”) and a saturation drain current of the E-type MOS transistor 55 (EnhTr 3) is output as a constant current value Iref.
In this case, a saturation drain current of a D-type MOS transistor 51 is a constant current source and the drain and the gate are common in the E-type MOS transistor 53. Therefore, the gate voltage is determined so as to be an upper part constant current value. Since the gate voltage is applied to the E-type MOS transistor 55 so that the E-type MOS transistor 55 operates, an electrical current value of the D-type MOS transistor is a current mirror.
In a constant current circuit having the D-type MOS transistor 51 (DepTr1), the E-type MOS transistor 53 (EnhTr1), and the E-type MOS transistor 55 (EnhTr 3), if manufacturing unevenness of the threshold voltage of the D-type MOS transistor 51 (DepTr1) is large, unevenness of values of the saturation drain electrical current flowing in the D-type MOS transistor 51 (DepTr1) becomes large and the saturation drain current (constant current value Iref) of the E-type MOS transistor 55 (EnhTr 3) is also drastically influenced.
In the D-type MOS transistor 51 (DepTr1), change of the threshold value voltage due to the temperature is changed. As a result of this, the standard voltage 57 made by threshold value difference, namely the difference of the threshold voltages of the D-type MOS transistor 51 (DepTr1) and the E-type MOS transistor 53 (EnhTr1), is unstable so that the entirety of the constant current circuit using this standard voltage 57 is unstable.
For example, Japanese Laid-Open Patent Application Publication No. 2-266407 discloses a technique for solving this problem. In this technique, the resistance is provided between the substrate and the source of the the E-type MOS transistor 55 (EnfTr 3) and trimming by laser rays is applied to this resistance, so that the electric current value is adjusted.
For example, Japanese Laid-Open Patent Application Publication No. 2004-192518 discloses a technique whereby a constant current generation circuit having small temperature dependency is realized by trimming the resistance.
However, in a case where trimming process is applied, a transistor having different size has to be prepared and a large area for making a bit for trimming easy is required. Therefore, correction cannot be made for the change of the temperature.
In addition, a stabilizing technique of the constant current circuit is discussed Japanese Patent No. 2599304, Japanese Laid-Open Patent Application Publication No. 4-97405, Japanese Patent No. 2800523, Japanese Laid-Open Patent Application Publication No. 2002-236521, Japanese Patent No. 3052818, and Japanese Laid-Open Patent Application Publication No. 7-160347.
Thus, it is a problem to be solved by the present invention that the change of the threshold value of voltage due to unevenness of manufacturing of the D-type MOS transistor 51 (DepTr1) and the change of the threshold value of voltage due to the temperature change are generated in the related art constant current circuit shown in FIG. 3 so that the constant current circuit shown in FIG. 3 is unstable.